Density dependent mortality in the springtail species Orchesella cincta due to predation by the carabid beetle Notiophilus biguttatus

Mortality of the collembole species Orchesella cincta (L.), due to predation by the carabid Notiophilus biguttatus F., has been measured in the laboratory at three levels of prey density. Prey densities were chosen such that the predator's consumption at the lowest prey density was a little above the maintenance ration. Between the two lowest prey densities, mortality increased significantly with prey density. We presume that density-dependent search activity by the predator is responsible for the positive density-dependent mortality. A method is proposed to solve the problem of decreasing prey density during the observations.

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Relation positive entre la densité de Orchesella cincta et la mortalité due à la prédation part Nothiophilus biguttarus

Résumé Cet article décrit l'importance de la prédation par le carabe N. biguttatus en fonction de la densité de la proie, O. cincta. Les résultats montrent que, sous l'effet de la prédation, la mortalité de la proie augmente avec sa densité. La quantité minimale de proies qui doit être consommée pour permettre la reproduction du carabe, a été déterminée. La relation entre la fécondité du carabe et l'effectif consommé, montre l'effet stabilisant de la prédation sur les populations de proies.La relation de dépendance de la densité observée avec la mortalité est due à une modification du comportement du carabe en fonction de la densité de sa proie. Nous proposons un modèle permettant de prévoir la régression de la population de proies sous les effets de la prédation.

     

Paternity Analysis in a Hexapod (Orchesella cincta; Collembola) with Indirect Sperm Transfer

In species where males and females interact during mating, the role of females in sexual selection cannot always be demonstrated unambiguously. Here we present a model system to study female choice for mates. Orchesella cinca is a soil-dwelling hexapod with indirect sperm transfer. Females and males do not interact physically for reproduction. We gave females the choice between spermatophores produced by two different males. Paternity analysis based on microsatellite variation revealed that offspring in one clutch were sired by one male only. Direct observations showed that after a female has taken up a spermatophore, the female's receptivity to further spermatophore uptake seem to end. Our results imply that the female is in full control of paternity.     

Distant Lovers: Spermatophore Deposition and Destruction Behavior by Male Springtails

Males of the springtail Orchesella cincta transfer their sperm indirectly using spermatophores. The present study aimed to investigate how individual males maximize their reproductive success. In an experiment examining the role of chemical communication, it appeared that the males deposit their spermatophores preferentially on patches which have been conditioned with conspecifics. Varying conditioning intensity resulted in a response which can be described by a saturation curve. In a comparison involving a related species the conditioning effect was strictly species specific. An experiment where the sexes were compared showed that conditioning with males exerts a slightly stronger effect than conditioning with females. In another experiment spermatophore depositing males showed a strong preference for patches with spermatophores of previous males, which they replaced with their own. An explanation for this may be that the males parasitize each other's scent marks. When a number of males are kept together they destroy most of each other's spermatophores. The destruction behavior is specifically displayed by males in their reproductive phase. Video recordings showed that deliberate destruction of spermatophores takes place by eating them and that the males are able to recognize and spare their own spermatophores.     

Components of predatory behaviour underlying density-dependent prey-size selection by Notiophilus biguttatus F. (Carabidae,Coleoptera)

Summary Prey-size selection by the carabid beetle Notiophilus biguttatus F. was studied, using prey belonging to two-size-classes of the spring-tail species Orchesella cincta (L.) (Collembola). The highest values for attack readiness and success ratio of the attacks were obtained for the smaller prey animals. A model for prey-size selection was constructed in which these two parameters are included, predicting an increasing chance of incorporating large prey in the diet with declining total prey density. This prediction was tested experimentally and qualitatively confirmed. In a quantitative sense, however, the results deviated from the expectation. This was caused by a greater attack readiness of the beetles in a semi-natural environment (in which the prediction was tested) than in the artificial environment in which the parameters had been estimated. General relevance of the results, their bearing on optimal foraging theory and their ecological implications are discussed.     

The lrp gene product regulates expression of lysU in Escherichia coli K-12.

In Escherichia coli K-12, expression of the lysU gene is regulated by the lrp gene product, as indicated by an increase in the level of lysyl-tRNA synthetase activity and LysU protein in an lrp mutant. Comparison of the patterns of protein expression visualized by two-dimensional gel electrophoresis indicated that LysU is present at higher levels in an lrp strain than in its isogenic lrp+ parent. The purified lrp gene product was shown to bind to sites upstream of the lysU gene and to protect several sites against DNase I digestion. A region extending over 100 nucleotides, between 60 and 160 nucleotides upstream from the start of the lysU coding sequence, showed altered sensitivity to DNase I digestion in the presence of the Lrp protein. The extent of protected DNA suggests a complex interaction of Lrp protein and upstream lysU DNA.     

Observations on life cycle and feeding ecology of two recently introduced predatory beetle species at South Georgia,sub-Antarctic

On South Georgia, two recently introduced species of predatory beetle,Oopterus soledadinus andTrechisibus antarcticus (Coleoptera, Carabidae), were studied in the period November 1991–April 1992. The study area comprised the coastal area around Stromness Bay, in particular the surroundings of the abandoned whaling station at Husvik. The study investigated the life cycle of both species and, forT. antarcticus, aspects of feeding. The occurrence of both teneral and gravid beetles was observed for the whole of the summer period.Trechisibus antarcticus appeared to be the more voracious predator of the two; its impact on other populations of soil animals may be large as shown by its effect on the endemic detritivorous beetleHydromedion sparsutum (Perimylopidae).     

NEGATIVE MATERNAL EFFECT REVISITED: A TEST ON TWO POPULATIONS OF ORCHESELLA CINCTA L. (COLLEMBOLA: ENTOMOBRYIDAE)

A Dutch population of Orchesella cincta had been demonstrated to exhibit a negative maternal effect on age at first reproduction, which caused alternation of short and long generations. The adaptive significance of such a mechanism was assumed to be associated with the bivoltine life cycle of Dutch O. cincta. We expected that it would be absent in a non bivoltine population sampled in Siena, Italy. To test this hypothesis we performed a parent-offspring regression experiment with both populations simultaneously. The experiment showed that there was no negative maternal effect in both populations. We leave open the question of the cause of the discrepancy between the previous result with the Dutch population and the present result. The results of our experiment were also used to determine heritabilities of the traits age, mass and number of molts at first reproduction, and size of the first clutch. The estimates of heritabilities were often not significantly different from zero, especially in the Italian population which had only one significant heritability.     

Mitochondrial genome sequence of <Emphasis Type="Italic">Unionicola parkeri</Emphasis> (Acari: Trombidiformes: Unionicolidae): molecular synapomorphies between closely-related <Emphasis Type="Italic">Unionicola</Emphasis> gill mites

The mitochondrial genome of Unionicola parkeri is a 14,734 bp circular DNA molecule. The sequence and annotation revealed a unique gene order, related to but distinct from the gene order in the closely related species U. foili. Mitochondrial tRNA sequences annotated in this genome predict non-canonical secondary structures for these molecules. The continuing patterns of unique gene orders and unusual tRNA structures in the Trombidiformes in general and Unionicola in particular support the use of phylogenetic approaches that use these types of molecular features as shared, derived character states. Further progress in using these molecular character states to reconstruct phylogeny will depend on careful annotation, especially of tRNA genes.     

The invasion of the carnivorous carabid beetle Trechisibus antarcticus on South Georgia (sub-Antarctic) and its effect on the endemic herbivorous beetle Hydromedion spasutum

Recently two species of carabid beetle were accidentally introduced onto the sub-Antarctic island of South Georgia. Both species are carnivorous and flightless. One of the species, Trechisibus antarcticus, is locally very abundant and in the process of invading the coastal lowland area, where the endemic herbivorous beetle Hydromedion sparsutum (Perimylopidae) is common. Field samples showed the abundance of the endemic species to be much lower, and its adult body size to be larger, in carabid-infested locations than in carabid-free locations. The sample data allowed us to estimate the growth rate of the H. sparsutum larvae and to reconstruct the most likely life-cycle of both species. A laboratory experiment showed a high mortality for the first three (out of six) larval instars of H. sparsutum in groups which had been subjected to predation by T. antarcticus. The duration of the period during which the larvae are vulnerable to predation was shown in a growth experiment to depend on food type. Quantitative and qualitative aspects of the interaction between the introduced predator and the endemic prey, and conditions which allowed the former to invade are discussed.